Mandevillas bloom all year long in warm sunny places. Its blooming season will depend on the environment it grows in and its care. They are hardy in USDA hardiness zones 9 through 11.
These plants are tropical, so they like air humidity and plenty of sunshine. They don’t bloom well in cold weather or shaded areas. That said, too much sun during summer can dry up the soil too fast and damage your plants. So, it is important to choose a sunny spot, but that offers a bit of protection during the hottest hours of the day, especially during summer.
It is normal for Mandevillas to lose some leaves and flowers after a drastic change in the environment. Plants need to get adapted to the environment they live in. It can be a nursery, the gardening department of a big-box store, or at your home. Once you bring them home, expect some changes, this is just the plant adapting to its new space and environment.
One of the most common reasons for these plants not to bloom or lose their flowers prematurely is underwatering and overwatering. The soil must drain out any excess water after watering, especially when planted in pots. If the soil gets saturated with water and doesn’t have where to go, their tuberous roots will eventually get damaged, and the first signs will be noticed in the flowers. These will turn brown, dry, soggy, or drop prematurely.
Mandevillas also need a good fertilizer to bloom. Any well-balanced fertilizer will do great, but if you want them to bloom profusely, look for one that is high in potassium.